Export Wavefront .mtl file -- Where is the reflectance coefficient?
I'm not sure this is directly related to Blender, or Blender's .obj exporter. The wavefront file description mentions a set of 11 illumination models (from 0 to 10), none of which seem to specify explicitly any coefficient for reflection, merely that there is reflection:


  Multiple illumination models are available, per material. These are
  enumerated as follows:

0. Color on and Ambient off
1. Color on and Ambient on
2. Highlight on
3. Reflection on and Ray trace on
4. Transparency: Glass on, Reflection: Ray trace on
5. Reflection: Fresnel on and Ray trace on
6. Transparency: Refraction on, Reflection: Fresnel off and Ray trace on
7. Transparency: Refraction on, Reflection: Fresnel on and Ray trace on
8. Reflection on and Ray trace off
9. Transparency: Glass on, Reflection: Ray trace off
10. Casts shadows onto invisible surfaces